In June 2010 six people began a simulation of a flight to Mars that will require them to remain in total isolation for 18 months. No one has ever been left alone like this-which is why the Mars500 project is considered one of the most dangerous experiments in human history.– Excerpt from [page 3,Feb 2011, Ideas and Discovery magazine]

A few reasons why it’s so dangerous :

Crew members in previous space missions developed animosity towards one another about halfway into the mission, and this often led to violent outbreaks. The first simulation of a mission to Mars had to be cancelled after 180 days because two crew members began fighting and a female crew member was nearly raped. The crew would have likely ended up dead if this was a real mission.

There are no Internet or telephones. All messages are sent to the “ground station” at the IBMP in Moscow. As the ship moves further and further away from the earth there is an increase in communication time delay. For example, after just two months, it would take 20 minutes for a distress call to reach the base and at least another 20 for the base’s response to be received. In an acute emergency- this delay could mean the difference between life and death.

There is a possibility that the isolation might lead to madness. Studies done with prisoners subjected to long periods of solitary confinement have shown that such isolations alters their hormonal balance and can lead to hallucinations.